Measuring On-chip resistive termination (impedance adaptation)

[steamedhams]

I am measuring the resistive termination of an FPGA's internal complex IO (single ended for now).
I've been wondering how to do this..

I was thinking, if I added an ammeter to the pad and then ground to the other side, i could see the current flow.. but doesn't this also draw current from the output buffer causing issues??

However, if I were to somehow deactivate the output buffer, then I am just creating a circuit from Vref/2 across the resistor and then to ground through my ammeter.

Seems simple enough but I am not really sure.

[NorthGuy]

Connect a 50 Ohm resistor between the pad and Vref (or between the pad and ground). Measure the voltage on the pad and calculate using Ohm's law.

[steamedhams]

wouldnt an ammeter be more accurate? why 50 ohm?

[NorthGuy]

wouldnt an ammeter be more accurate?

I don't think so. It also measures voltage difference across a resistor, but it's resistor is much smaller than yours.

why 50 ohm?

Mathematically, you'll get the least error if both DUT resistor and yours are the same.

[steamedhams]

turns out i had the correct approach just connected to the wrong pad.